This invention relates to the production of reaction-bonded silicon carbide bodies.
Reaction-bonded silicon carbide bodies are produced by reaction sintering of a coherent mixture (or green body) of carbon and silicon carbide powders in the presence of molten silicon (hereinafter referred to as "siliconising") whereby the carbon in the mixture is converted to bonding silicon carbide and a substantially continuous silicon carbide matrix is formed in a substantially continuous free silicon phase.
In one method of siliconising molten silicon is drawn upwards through a green body of carbon and silicon carbide by capillary action. Porosity has accordingly to be provided in that body to allow infiltration of the molten silicon. The rate of climb of the molten silicon through such a green body is proportional to the pore size in the body and inversely proportional to the height reached. The greater the porosity, therefore, the more rapid the infiltration of silicon and the completion of the conversion of all the carbon in the green body to bonding silicon carbide. However, the greater the porosity which is provided, the higher the proportion of silicon in the reaction-sintered body, and the presence of an excess of free silicon in the body may have a deleterious effect on the desired properties of the reaction-sintered body. On the other hand, if the porosity is insufficient, the infiltration may be inhibited to such an extent that conversion of carbon to silicon carbide is incomplete or impracticably slow and in considering this factor account has to be taken not only of the initial porosity but of the increase in molecular volume when carbon is converted to silicon carbide so that there is a tendency for the newly formed, bonding silicon carbide to fill the available porosity and block off the flow of molten silicon through the body. Reducing the proportion of carbon in the green body and hence the formation of bonding silicon carbide does not necessarily provide a satisfactory solution however, because, in general, the lower the proportion of carbon in the green body the higher the proportion of free silicon present after siliconising.
The introduction of additional porosity in the outer layer of a green body by reaction of the carbon therein with silicon monoxide vapour has already been described, for example, in UK Patent Specification No. 1,180,918. By this method surface porosity can be enhanced from about 10% up to 40% and the surface pore size from about 1 micron to up to 10 microns. The rate of climb is thus increased by about an order of magnitude and, because silicon can now infiltrate the body laterally as well as vertically a 450 mm length can be siliconised in about four hours but double the length (900 mm) would however take four times as long (at least 16 hours). This may be commercially unacceptable. The present invention seeks to provide means whereby the rate of climb by molten silicon may be further accelerated and also to avoid or reduce the gradation in free silicon content which may be a disadvantageous result of the production of reaction-sintered bodies using the above method.
It has already been proposed in U.S. Pat. No. 4,301,132 that composite green bodies should be produced in which one portion contains a lower proportion of carbon to silicon carbide than a second portion. If the portion containing the lower proportion of carbon is a layer or coating on a surface of the second portion it may, by suitable selection of the porosity of that layer or coating, provide a pathway for molten silicon to the upper parts of the composite body whereby the silicon reaches those upper parts more readily than by passage through the second portion. Such silicon can then infiltrate the second portion laterally in contrast to the substantially vertical alternative route through the second portion. After siliconising the surface layer or coating, containing an excess of free silicon, may be removed from the second portion. One way of doing this is to leach out the free silicon with alkali to leave, on the surface of the second portion, a reaction-sintered silicon carbide skeleton which can be machined away.